Variable OFDM subchannel coding and modulation

ABSTRACT

A system for selecting a modulation scheme and an error correction coding scheme for each subchannel in an OFDM system based on the energy detected on that subchannel.

BACKGROUND OF INVENTION

1. Field of Invention

The invention relates generally to wireless communication and moreparticularly to a system for efficiently using OFDM subchannels.

2. Discussion of Related Art

Frequency Division Multiplexing (FDM) is a well known process by whichmultiple signals are modulated on different frequency carrier waves. FDMhas been used for decades in radio and television broadcast. Radio andtelevision signals are sent and received on different frequencies, eachcorresponding to a different “channel.”

Orthogonal Frequency Division Multiplexing (OFDM) has also been known inthe art at least since the late 1960's. In OFDM, a single transmittertransmits on many different orthogonal frequencies simultaneously.Orthogonal frequencies are frequencies that are independent with respectto the relative phase relationship between the frequencies. In OFDM, theavailable bandwidth is subdivided into a number of equal-bandwidth“subchannels.” OFDM is advantageous for wireless communication becauseit reduces interference or crosstalk between signal transmissions,ultimately permitting data transmission at higher throughput with fewererrors. OFDM is also known as Discrete Multitone Modulation (DMT). OFDMis employed in many standards used today for wireless communication. Forexample, both the IEEE 802.11 a wireless LAN standard and the 802.11 gwireless LAN standard rely on an implementation of OFDM for signaltransmission. One early reference describing OFDM is R. W. Chang,Synthesis of band-limited orthogonal signals for multi-channel datatransmission, Bell System Technical Journal (46), 1775-1796 (1966).

OFDM thus functions by breaking one high speed data stream into a numberof lower-speed data streams, which are then transmitted in parallel(i.e., simultaneously). Each lower speed stream is used to modulate asubcarrier. This creates a “multi-carrier” transmission by dividing awide frequency band (or channel) into a number of narrower frequencybands (or subchannels), each modulated with a signal stream. By sendingmultiple signal streams simultaneously, each at a lower rate,interference such as multipath or Raleigh fading can be attenuated oreliminated without decreasing the overall rate of transmission.

SUMMARY OF INVENTION

This Summary provides an illustrative context for aspects of theinvention, in a simplified form. It is not intended to be used todetermine the scope of the claimed subject matter. Aspects of theinvention are described more fully below in the Detailed Description.

Described herein are systems and methods for selecting a modulationscheme and an error correction coding scheme for each subchannel in anOFDM system based on the energy detected on that subchannel.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a spectrum diagram showing the subdivision of the channelbandwidth to be used into several subchannels of equal width.

FIG. 2 is a block diagram of a multi-carrier OFDM digital communicationsystem.

FIG. 3 is a flow diagram illustrating one embodiment of the invention.

FIG. 4 is a diagram of a system that implements some aspects of theinvention.

DETAILED DESCRIPTION

This invention covers a novel use of OFDM subchannels. According to theclaimed invention, each OFDM subchannel may be modulated with adifferent modulation scheme and/or error correction coding schemespecifically suited to the characteristics of that subchannel. Theinvention may be implemented in hardware or software, or somecombination thereof. Embodiments include a system, a method, andinstructions stored in a computer-readable medium.

Computer readable media can be any available media that can be accessedby a computer. By way of example, and not limitation, computer readablemedia may comprise computer storage media and communication media.Computer storage media includes volatile and nonvolatile, removable andnon-removable media implemented in any method or technology for storageof information such as computer readable instructions, data structures,program modules or other data. Computer storage media includes, but isnot limited to, RAM, ROM, EEPROM, flash memory or other memorytechnology, CD-ROM, digital versatile disks (DVD) or other opticalstorage, magnetic cassettes, magnetic tape, magnetic disk storage orother magnetic storage devices, other types of volatile and non-volatilememory, any other medium which can be used to store the desiredinformation and which can accessed by a computer, and any suitablecombination of the foregoing.

The computer-readable media may be transportable such that theinstructions stored thereon can be loaded onto any suitable computersystem resource to implement the aspects of the present inventiondiscussed herein. In addition, it should be appreciated that theinstructions stored on the computer-readable medium, described above,are not limited to instructions embodied as part of an applicationprogram running on a host computer. Rather, the instructions may beembodied as any type of computer code (e.g., software or microcode) thatcan be employed to program a processor to implement the aspects of thepresent invention discussed below.

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” or “having,”“containing,” “involving,” and variations thereof herein, is meant toencompass the items listed thereafter and equivalents thereof as well asadditional items.

As shown in FIG. 1, in OFDM, the available channel bandwidth W issubdivided into a number of equal-bandwidth subchannels. Each subchannelis sufficiently narrow so that the frequency response characteristics ofthe subchannel are nearly ideal. The number of subchannels is the totalavailable bandwidth divided by the bandwidth of each subchannel. Thenumber of subchannels K can thus be expressed as:$K = \frac{W}{\Delta\quad f}$

Each subchannel k has an associated carrier wave. This carrier wave canbe expressed as:x _(k)(t)=sin 2πf _(k) t

Where x_(k)(t) is the carrier wave for subchannel k as a function oftime t. f_(k) is the mid-frequency of subchannel k, and k ranges from 0to K−1.

The symbol rate 1/T is set for each subchannel to be equal to theseparation Δ_(f) of adjacent subcarriers. The subcarriers will thus beorthogonal over the symbol interval T, independent of the relative phaserelationship between subcarriers. This relationship can be expressed as:∫₀^(T)sin   (2π  f_(k)t + ϕ_(k))  sin   (2π  f_(j)t + ϕ_(j))𝕕t = 0

Where f_(k)−f_(j)=n/T, n=1, 2, . . . , independent of the values of thephases Φ_(k) and Φ_(j).

In an OFDM system, the symbol rate on each subchannel can be reducedrelative to the symbol rate on a single carrier system that employs theentire bandwidth Wand transmits data at the same rate as the OFDMsystem. Hence, the symbol interval T (the inverse of the symbol rate) inthe OFDM system can be expressed as:T=KT _(s)

Where T_(s) is the symbol interval of a single-carrier system employingthe entire bandwidth W and transmitting data at the same rate as theOFDM system. For example, if the symbol rate across the entire bandwidthfor one channel is 72 million symbols per second, and the channel isdivided into 48 subchannels, each subchannel would only need to carry1.5 million symbols per second to achieve the same total data rate. Thislower symbol rate reduces inter-symbol interference and thus mitigatesthe effects of multipath fading. Accordingly, OFDM provides for superiorlink quality and robustness of communication.

In an OFDM system, the transmitter receives input data in the frequencydomain and converts it to a time domain signal. A carrier wave ismodulated by the time domain signal for wireless transmission. Thereceiver receives the signal, demodulates the wave, and converts thesignal back to the frequency domain for further processing.

A simplified OFDM system is illustrated in FIG. 2. In the illustratedembodiment, the input data stream 201 is provided by the application tothe OFDM transmitter 200. In a standard TCP/IP communications stack,this data could be received at the physical layer or data link layer;however, the invention is not limited to any particular source of dataor mechanism for providing the data to the transmitter, and could beimplemented in hardware or software, and at various layers of thenetwork stack. The input data stream 201 is received by aserial-to-parallel buffer 202. The serial-to-parallel buffer 202 breaksthe serial data stream up into several parallel data streams. The numberof parallel data streams is equal to the number of subchannels availablefor OFDM broadcast, or K as used above.

In one embodiment, the serial-to-parallel buffer 202 divides theinformation sequence received from input data 201 into frames of B_(f)bits. The B_(f) bits in each frame are parsed into K groups, where theith group is assigned b_(i) bits. This relationship may be expressed as:${\sum\limits_{i = 1}^{K}b_{i}} = B_{f}$

Each of the parallel data streams generated by the serial-to-parallelbuffer 202 is then sent to a multicarrier modulator 203. Themulticarrier modulator 203 modulates each OFDM subcarrier with each ofthe parallel data streams. The multicarrier modulator 203 can beefficiently implemented by use of the Inverse Fast Fourier Transformalgorithm to compute the time domain signal, although any algorithm maybe used that converts a frequency domain signal to a time domain signal.

The multicarrier modulator 203 may use any modulation scheme to modulateeach of the incoming data streams. In a preferred embodiment, thesignals are modulated with quadrature amplitude modulation (QAM). AnyQAM constellation may be used. For example, the modulator may use16-QAM, 64-QAM, 128-QAM or 256-QAM. A modulation scheme may be selectedbased on the required data rate, the available subchannels, the noise oneach subchannel, or other factors. Each subchannel may use a differentconstellation, depending, for example, on the noise on that subchannel.The novel claimed system for selecting a different modulation scheme anderror correction scheme claimed in this patent is discussed below.

In this example, the multicarrier modulator 203 thus generates Kindependent QAM subchannels, where the symbol rate for each subchannelis 1/T and the signal in each subchannel has a distinct QAMconstellation. According to this example, the number of signal pointsfor the ith subchannel can be expressed as:M _(i)=2^(b) ^(i)

The complex-valued signal points corresponding to the informationsignals on each of the K subchannels can be represented as X_(k), wherek=0, 1, . . . , K−1. These symbols X_(k) represent the values of theDiscrete Fourier Transform of a multicarrier OFDM signal x(t), where themodulation on each subcarrier is QAM. Since x(t) must be a real-valuedsignal, its N-point Discrete Fourier Transform X_(k) must satisfy thesymmetry property. Therefore, the system creates N=2K symbols from Kinformation symbols by defining:X _(N−K) =X* _(K) , k=1,2, . . . , K−1X′ ₀=Re(X ₀)X _(N)=Im(X ₀)

Here X₀ is split into two parts, both of which are real. The newsequence of symbols can be expressed as X′_(k), where k=0, 1, . . . ,N−1. The N-point Inverse Direct Fourier Transform for each subchannelx_(n) can thus be expressed as:${x_{n} = {{\frac{1}{\sqrt{N}}{\sum\limits_{k = 0}^{N - 1}{X_{k}^{\prime}{\exp\left( {j\quad 2\pi\quad{{nk}/N}} \right)}\quad n}}} = 0}},1,\ldots\quad,{N - 1}$

In this equation, $\frac{1}{\sqrt{N}}$is a scale factor. The sequence x_(n) where 0<=n<=N−1 thus correspondsto samples of the multicarrier OFDM signal x(t), consisting of Ksubcarriers.

A cyclic prefix, which acts a guard interval, is added to each of theparallel modulated waves at 204. This guard interval insures that thesubchannels will remain orthogonal, even if multipath fading causes thesubcarriers to arrive at the receiver with some delay spread. Theparallel streams with the cyclic prefix are then merged back into asingle serial stream at 204. Finally, the digital data stream isconverted to an analog signal 205, and output for wireless transmission.

The transmitted signal can be received by the receiver 210 and processedto recover the original data stream. First, the analog signal isconverted back to a digital signal by an analog to digital converter211. The cyclic prefix is removed and the separate subcarriers areconverted back to separate streams at 212. Each parallel data stream isdemodulated by a multicarrier demodulator 213, preferably with a FastFourier Transform algorithm. Finally, at 214 the parallel streams arereassembled into a single serial stream and output to the receivingdevice 215.

A key inventive aspect of this system that will be appreciated by one ofordinary skill in the art is the method for separately selecting themodulation scheme and error correction coding scheme to be used on eachsubchannel. One example of this method is illustrated in FIG. 3.

FIG. 3 depicts a flowchart illustrating one preferred embodiment of theinvention. FIG. 3 illustrates a process that can be utilized by thetransmitter to select a modulation scheme and error correction codingscheme to be used for each subchannel. This process could be implementedin hardware or software.

The application or operating system requests a particular data rate fortransmission at 301. In this embodiment, the system 300 then followsseveral steps to select the optimal modulation scheme and errorcorrection coding scheme to achieve this requested data rate.

The system starts with a threshold vector that correlates a modulationscheme and error correction coding scheme with a detectedsignal-to-noise ratio. This vector can be expressed as:Θ={θ₁,θ₂, . . . ,θ_(s)}

Each value in the Θ vector is a signal-to-noise ratio (or energy level)and a

corresponding modulation scheme and error correction coding scheme. Byway of illustration, θ₁ could be set at +20 dB, θ₂ could be set at 0 dB,and θ₃ could be set at −20 dB. The threshold vector sets a differentmodulation scheme and error coding scheme to correspond with each ofthese signal-to-noise ratios. The modulation scheme could be anymodulation scheme, including, for example, the well known schemes ofquadrature amplitude modulation (QAM), quadrature phase shift keying(QPSK), binary phase shift keying (BPSK), or any other scheme. Likewise,the error coding scheme is not limited to any particular coding scheme,but could include Reed-Solomon coding, convolutional coding, Viterbicoding, or any other coding scheme. The vector will be optimized so thatthe modulation scheme and error correction coding scheme are appropriatefor the corresponding signal-to-noise ratio.

In one embodiment of the invention, a base error correction codingscheme can be selected to apply to all OFDM subchannels. Higher errorrate codes are then punctured from the base code and can be selected foreach subchannel based on the noise detected on that subchannel.Puncturing is the process of removing some of the parity bits after anerror correction code has been applied. Because there is less redundancywith some parity bits removed, puncturing has the same effect asencoding with a higher rate error correction code. Puncturing allows thesame decoder to be used, regardless of how many bits have beenpunctured, and thus considerably increases the flexibility of the systemwithout significantly increasing its complexity.

At 302, the first subchannel from the available spectrum to be used isselected. The transmitter, optionally with feedback from the receiver,detects the energy level on that subchannel at 303. The transmitter thenselects the modulation scheme and error correction scheme for theselected subchannel by comparing the detected energy level to the valuesin the threshold vector at 304. If the detected energy level fallsbetween two values in the threshold vector, the corresponding modulationscheme and error correction coding scheme for the lower threshold valueis selected. Expressed mathematically: if θ_(j)<E_(i)<θ_(j+1), thenmodulation scheme m_(j) and error correction coding rate r_(j) areselected. The transmitter then selects the next subchannel for testingat 305. This process is repeated from 303 to 305 until all subchannelshave an associated modulation scheme and error correction coding schemebased on each subchannel's signal-to-noise ratio.

Once a modulation scheme and error correction coding scheme has beenselected for each OFDM subchannel, it is possible to calculate a totaldata rate across all of the subchannels. The system can then check at306 whether that total data rate exceeds the required rate provided bythe application. If it does, the system has two nonexclusive options. Itcan select a lower order modulation scheme for one subchannel at 307, orit can select a lower rate error correction coding scheme for onesubchannel at 308. If the system selects a lower order modulationscheme, it can select that scheme for the subchannel that has thehighest order modulation scheme, or that has the highest signal-to-noiseratio, or could select any arbitrary subchannel. Similarly, the systemcould select a lower rate error correction coding scheme for thenoisiest subchannel or any other subchannel. The total data rate withthe new modulation scheme or error correction coding scheme is thenrecalculated, and steps 306 to 308 are repeated until the total rate isequal to the required rate.

If, on the other hand, the required rate exceeds the total calculatedrate of transmission at 309, the system must adjust the threshold vectorat 310 such that the modulation schemes and error correction codingschemes selected for each noise level will provide a faster ratetransmission. The process from 302 to 305 is then repeated with the newthreshold vector.

FIG. 4 illustrates another embodiment of the invention. This figureshows a system 400 that that accepts a data rate from an application 401and provides information to an OFDM transmitter 405 as to how it willtransmit data. The system 400 comprises an energy detection module 402,a modulation selection scheme module 403, and an error correction codingscheme module 404. The energy detection module 402 detects the noiselevel on each OFDM subchannel. The data gathered by the energy detectionmodule 402 are provided to the modulation selection scheme module 403 aswell as the error correction coding scheme module 404, which eitherindependently or in concert will select a modulation scheme and an errorcorrection coding scheme respectively. The selected schemes are thenprovided by the system 400 to the transmitter 405, which can then begintransmitting over OFDM using those schemes.

In yet another embodiment, the invention relates to a computer-readablemedium having computer-executable instructions for performing steps. Thesteps include measuring the signal-to-noise level on each OFDMsubchannel and coding a modulated signal on each OFDM subchannel usingan error correction coding scheme and modulation scheme based on thesignal-to-noise level measured for each OFDM subchannel.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of wireless communication between two or more devices at aminimum data rate, the method comprising the acts of: a) detecting theenergy level on one or more subchannels; and b) selecting a modulationscheme for each of said subchannels based on the energy level detectedfor each of said subchannels.
 2. The method of claim 1, furthercomprising selecting an error correction coding scheme for each of saidsubchannels based on the energy level detected for said subchannel. 3.The method of claim 2, further comprising: c) calculating the totaltransmission rate based on the selected modulation scheme and theselected error correction coding scheme; and d) selecting a higher ordermodulation scheme on one subchannel if the minimum data rate exceedssaid total transmission rate.
 4. The method of claim 2, furthercomprising: c) calculating the total transmission rate based on theselected modulation scheme and the selected error correction codingscheme; and d) selecting an error correction coding scheme with a highererror correction rate on one subchannel if the minimum data rate exceedssaid total transmission rate.
 5. The method of claim 1, wherein themodulation scheme is quadrature amplitude modulation.
 6. The method ofclaim 3, wherein the modulation scheme for each of said subchannels isselected from 16-QAM, 64-QAM, 128-QAM, 256-QAM, 512-QAM, and 1024-QAM.7. The method of claim 2, further comprising: c) transmitting data oversaid subchannels using orthogonal frequency division multiplexing. 8.The method of claim 2, wherein the error correction coding scheme is aconvolutional coding scheme.
 9. The method of claim 2, wherein the errorcorrection coding scheme is a block coding scheme.
 10. The method ofclaim 2, wherein the error correction coding scheme is selected from aseries of coding schemes, such that each coding scheme is punctured froma base coding scheme.
 11. A wireless communication system fortransmitting data at a requested rate including a plurality ofcommunication devices, the system comprising: a) an energy detectionmodule to detect the noise level on one or more OFDM subchannels; and b)a modulation selection scheme module to select a modulation scheme foreach OFDM subchannel based on the energy level detected for thatsubchannel.
 12. The system of claim 11, wherein the system furthercomprises an error correction selection scheme module to select an errorcorrection scheme for each OFDM subchannel based on the energy leveldetected for that subchannel.
 13. The system of claim 11, wherein themodulation selection scheme module selects a quadrature amplitudemodulation scheme.
 14. The system of claim 11, wherein the quadratureamplitude modulation scheme is selected from 16-QAM, 64-QAM, 128-QAM,256-QAM, 512-QAM, and 1024-QAM.
 15. The system of claim 11, wherein theenergy detection module is part of a transmitter.
 16. The system ofclaim 15, wherein the energy detection module detects the noise levelusing feedback from a receiver.
 17. A computer-readable medium havingcomputer-readable signals stored thereon that define instructions that,as a result of being executed by a computer, instruct the computer toperform a method of wireless communication at a requested data rate, themethod comprising: a) measuring the signal-to-noise level on each OFDMsubchannel; and b) coding a modulated signal on each OFDM subchannelusing an error correction coding scheme based on the signal-to-noiselevel measured for each said OFDM subchannel.
 18. The computer-readablemedium of claim 15, wherein the method further comprises modulating asignal on each OFDM subchannel with a modulation scheme based on thesignal-to-noise level measured for each said OFDM subchannel.
 19. Thecomputer-readable medium of claim 16, wherein the modulation scheme isselected from quadrature amplitude modulation, quadrature phase shiftkeying, and binary phase shift keying.
 20. The computer-readable mediumof claim 15, wherein the error correction coding scheme is selected froma convolutional coding scheme and a block coding scheme.